1. Field of the Invention
This invention relates to a wire for use as overhead lines in electric railways, and a method of producing the same.
2. Prior Art
It is known that overhead lines for electric railways include in general contact wires for supplying electric power to electric rolling stocks, messenger wires for supplementing power to the electric rolling stocks and for supporting the contact wires in air, and auxiliary messenger wires for supporting the messenger wires.
These wires have conventionally been formed of pure copper or copper alloys containing 0.3 percent by weight Sn.
As is seen in super-express railways such as the Shinkansen, higher speed performance is increasingly required of electric rolling stocks manufactured in recent years, and an increase in wire tension is required of the wires. Accordingly, wires having higher tension are demanded.
To meet such demand, recently, copper alloy wires containing Cr and Zr and having a fundamental composition of the precipitation hardening type have been proposed for use as a wire having high tension. For example, in Japanese Provisional Patent Publications (Kokai) Nos. 3-56632 and 3-56633, there have been proposed wires each formed of a copper alloy having a chemical composition containing, by weight percent (hereinafter referred to "%"), 0.001 to 0.35% Zr, and 0.01 to 1.2% Cr, and if required, further containing 1.5% or less at least one element selected from the group consisting of 0.3% or less Mg, 1.5% or less Zn, 0.2% or less Ag, 0.5% or less Cd, and the balance of Cu and inevitable impurities including Sn, Si, P, Fe, Ni, Pb, As, Sb, Bi and Si whose contents are limited as follows: Sn: 100 ppm or less; Si: 50 ppm or less; P: 50 ppm or less; Fe: 100 ppm or less; Ni: 100 ppm or less; Pb: 20 ppm or less; As: 20 ppm or less; Sb: 20 ppm or less; Bi: 20 ppm or less; and Si: 10 ppm or less.
These wires formed of the copper alloys containing Cr and Zr are manufactured in the following manner: First, a copper alloy ingot having a predetermined composition is prepared, and the prepared alloy ingot is hot rolled or hot extruded at a temperature of 700.degree. to 850.degree. C. to produce a roughly rolled coil of pure copper or a copper alloy having a large diameter and a short length, followed by solution treatment thereof. Thereafter, cold drawing and aging treatment are repeated, to thereby effect wire drawing to a predetermined size. Thus, the wires are manufactured (see Japanese Patent Publications (Kokoku) Nos. 60-53739, 63-3936, etc.)
In recent years, however, it is not unusual for newly manufactured electric rolling stocks to have a speed as high as 350 kph or more. Accordingly, in order to ensure stable sliding contact of a pantograph of an electric rolling stock with a contact wire, it is required that the wire tension of the contact wire and the messenger wire be made larger than conventional wires and the wires of contact line (formed of a contact wire, a messenger wire, and an auxiliary messenger wire) be made lighter in view of the wave propagation velocity. However, none of the above-mentioned known wires are fully satisfactory in tensile strength, and therefore., wires more excellent in mechanical strength have been desired.
More specifically, in conventional wires of contact line which were previously formed of a copper contact wire and a messenger wire of a hard copper strand, a steel-cored copper contact wire having the same cross sectional area as the conventional copper contact wire has been used in place of the copper contact wire in recent years. As a result, the power-feeding capacity of the contact wire has decreased, whereby the messenger wire is required to share an increased rate of feeding of electric power (by about 60% or larger) than before to compensate for the decreased power-feeding capacity of the contact wire. Further, in these years, the power consumption per electric rolling stock has increased in electric railways, and the number of electric rolling stocks has also been increased.
On the other hand, since electric rolling stocks run faster, it is required that the whole wires of contact line be made lighter in weight in order that electric rolling stocks can stably collect power, in view of the wave propagation velocity. Messenger wires have thus been rendered smaller in diameter, e.g. a messenger wire formed of 7 fine wires each having a diameter of 4.3 mm has been replaced by one formed of 7 fine wires each having a diameter of 3.7 mm. Accordingly, since a larger amount of current than before flows through the messenger wire, the amount of heat generation thereof has become larger. To cope with the above problems, materials for messenger wires are demanded, which are excellent in tensile strength as well as a thermal creep resistance up to 200.degree. C. or 300.degree. C.
Messenger wires are maintained taut by their own tension obtained by weights having a weight of about 1000 kg and vertically hung at both ends of the wire. However, as electric rolling stocks pass, a repeated bending stress is applied to the ends of the wire. If the stress applied to the ends occurs tens of thousands of times, rupture would occur at the ends of the wire. Therefore, ends of messenger wires are required to be excellent in 90 degree repeated bending properties.
Further, a wire which is poor in pressure weldability suffers from rupture at a pressure welded portion thereof or in the vicinity thereof. Furthermore, if the tensile strength at the pressure welded portion is low, the wire is sometimes cut at the pressure welded portion, which can cause an accident.